Multi-kW-class 1.64 μm Er<sup>3+</sup>:YAG lasers based on heat-capacity operation [Invited]

Marc Eichhorn

doi:10.1364/OME.1.000321

1. Introduction

Resonantly diode-pumped Er³⁺:YAG lasers are promising candidates for various ”eye-safe” emissions in the 1.6 μm range with applications in lidar, free-space communication and illumination. These lasers were, however, limited in average output power dut to their quasi-three-level lasing schemes [1], being thought of as inefficient due to thermal reabsorption from the lower laser level. Therefore, high-power and high-energy applications including directed energy were thought of to be out of reach for ”eye-safe” solid-state lasers. In this context, the term ”eye-safe” of-course relates only to the fact that the laser wavelength is > 1.4 μm and therefore said to be in the ”eye-safer” wavelength range. This only means that this radiation cannot be focused onto the retina and thus a higher amount of irradiation is allowed than for non-eye-safe wavelengths. In terms of possible applications and certain fields of use it is a political and jurisdictional need. But, of-course, this does not reduce the direct laser hazard presented by a kW-class beam.

It has however been shown that highly effcient lasing in Er³⁺:YAG is possible already in 2006 using fiber-laser pumping with very high overall efficiencies of 73.5% [2]. Direct diode-pumping was performed even before in 2005 [3], but with reduced efficiency due to the broader spectral emission of diodes compared to fiber lasers. The strong difference in efficiency between these two cases shows how important it is to pump a quasi-three-level laser with high spectral and spatial brightness in order to obtain high efficiency and output power [1]. However, unlike in some fluoride hosts which are less hard optical materials and which show emission wavelengths unsuitable for good atmospheric transmission, most robust laser hosts that meet this atmospheric transmission criterion like YAG show quite strong positive thermal lensing. This makes it difficult to obtain high output powers without a strong degradation in beam quality [4].

The goal was to realize a robust ”eye-safe” high-power laser without the need for combining multiple lasers (ruling out coherent or spectral combining of fiber lasers). Keeping in mind in addition that the quantum-mechanical structure of Er³⁺ is extremely sensitive to upconversion losses even at moderate doping concentrations, also disk lasers can be ruled out unless cryogenic cooling might be used. However, this causes severe narrowing in pump absorption linewidth, increases laser emission cross sections that might cause amplified spontaneous emission problems and add in general a high complexity to the laser system. Therefore, only a solid block of laser medium can be used and the well-known thermal lens effects occurring in an actively cooled thermal management have to be avoided. A way to overcome this limitation, at least for a certain lasing time, is the heat capacity mode of operation of a solid-state laser [5] that reduces thermal lensing to a great extent and allows efficient laser operation on a time scale of seconds. The main idea of this concept is that without heating and under the condition of a very homogeneous excitation and extraction of the laser medium no thermal gradients and thus no thermal lens or birefringence will build up. As thermal lens and thermal birefringence is strongly reduced it is even possible to use much higher pump powers to operate the laser far above threshold and thus get laser output powers that would be impossible in an actively cooled laser medium due to crystal fracture or thermal instabilities of the laser resonator.

The first but non-eye-safe SSHCL was developed at Lawrence Livermore National Laboratory using flashlamp pumped Nd³⁺:GGG slabs [6], which is nowadays based on Nd³⁺:YAG pumped by high-power laser diodes, increasing laser efficiency and operation time. The laser obtains an optical-to-optical efficiency of ≈ 25% and an average output power of 67 kW [7]. However, as soon as the laser emission needs to be “eye-safe” in terms of its wavelength, the laser media are quasi-three-level laser media and therefore suffer from thermal reabsorption, making most of them unuseful for heat-capacity operation. Therefore thorough spectroscopic and numerial studies [8, 9] were necessary in order to discover that Er³⁺:YAG is a potential candidate for an eye-safe high power laser in heat-capacity mode and should only show a moderate to low power drop with crystal temperature. Using extremely-low-doped crystals (doping < 1%) then eliminates upconversion and also assures to obtain a high specific heat comparable with undoped YAG. A first small-scale prototype used to verify the concept itself, pumped by only 496 W, allowed already output powers of 70 – 125 W and operation times of several seconds [1, 10, 11]. Encouraged by these results further upscaling allowed to obtain a multi-kW-class Er³⁺:YAG SSHCL pumped with > 10 kW. This paper discusses the spectroscopic fundametals, the scaling properties and experimental results of this quasi-three-level resonantly diode-pumped Er³⁺:YAG SSHCL technology.

2. Theoretical background

2.1. Laser medium and spectroscopy

The most important points that make a quasi-three-level laser medium suitable for heat-capacity operation are a direct consequence of its quantum structure. A lower laser level far above the ground-state level is needed to decrease the influence of temperature on reabsorption, a well-suited emission from a low-lying state in the upper laser manifold ensures laser operation with lower inversion and an equal spacing of levels in the lower and upper laser manifold on some levels can result in multiple pump lines which may be addressed with one pump wavelength, resulting also in a low dependence of pump absorption on temperature. All these points are fulfilled in Er³⁺:YAG and in addition enhanced by an energy gap in the lower and upper laser manifold [1, 8]. The energy gap in the lower laser manifold ensures low reabsoption and high pump level population even at high temperatures, and the energy gap in the upper laser manifold concentrates the population onto the laser emitting level.

Another important point for high power operation is excited-state absorption (ESA). Again as a major benefit of Er³⁺:YAG, neither the CW operation line at 1645 nm, nor the high-inversion (Q-switch) operation line at 1617 nm is affected by ESA. However, some other emission lines, which are not important for laser operation, do show a direct overlap with ESA lines and therefore contribute to resonant upconversion [9, 12].

As already mentioned upconversion can be a strong loss mechanism in Er³⁺:YAG and thus needs to be avoided. Due to the strong r ⁻⁶ dependence of dipolar energy transfer on inter-ion distance r the best (and only) way to reduce or suppress upconversion is low doping. This results in doping concentrations of ≤ 0.5% and therefore in very long pump absorption lengths of generally 60 – 160 mm or beyond. On the one side this is an advantage for SSHCL operation as a dilute laser medium can accept a large amount of heat while showing a low temperature rise. Keeping in mind that the pump saturation intensity in Er³⁺:YAG is around 3.35 kW/cm², even very high-power lasers therefore will operate with rod-like elongated media. On the other side, however, especially for high-power diode-pumped operation, the spatial brightness and thus the divergence of multi-stack laser diode radiation is usually not sufficient to ensure a homogeneous excitation and to keep a high pump intensity allover the laser medium length which is important for high-efficiency operation [1]. In order to nevertheless keep such high pump intensities inside the laser medium, the laser rod needs to be polished on its periphery to trap the pump light by total internal reflection (TIR). However, using this technolgy care has to be taken in order not to generate ASE or, being more precise, parasitic lasing inside the laser medium. Also, a part of the fluorescence of the laser medium itself will be trapped and has to be taken into account. This fluorescence trapping, under the assumption that parasitic lasing can be avoided, is even beneficial for the laser efficiency as will be discussed later.

2.2. Pump delivery

The main objective of a pump delivery system for an SSHCL is to obtain a high degree of homogenization of the pump intensity while having the smallest possible loss on pump power. Conditioning of laser diode radiation for pumping purposes using non-imaging lens ducts has already been prooven to be useful [13]. However, these devices show some drawbacks in high-power operation: They need to have special anti-reflection coatings, especially at the exit window where large incidenct angles occur. As they have to be closely positioned to the laser medium ends, they do not allow passing the high-power laser beam, especially in an dual-end-pumped geometry. The smallest amount of background absorption would cause a thermal lens being created inside the duct. Therefore, a new pumping delivery scheme based on hollow ducts needed to be implemented, see Fig. 1. Based on a new theory [14] the right choice of geometry and materials can be made in order to obtain pump-light compression and homogenization at very high transmission values of the total pump-delivery system of > 90%. In the special case of the high-power kW-class laser presented here, a total transmission of even 98% is reached.

Fig. 1 Schematic of a hollow duct and its mathematical description by the equivalent surface.

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In this context it is very important to discuss the scaling properties of these pump-delivery systems. As in an SSHCL the pump and laser intensity should be kept constant under geometrical upscaling, the necessary pump power and thus the laser output power scales with the cross-sectional area of the laser medium. The scaling properties of the duct therefore need to be discussed in terms of geometrical scaling. Using the notation of the duct theory in [14], it is assumed that ã, b̃, ã′, b̃′ and L̃_D are the geometric dimensions of a duct having the transmission efficiency η̃_T while its entrance surface is completely covered by laser diodes. This stacked array is described by the lateral dimensions ã_d and b̃_d. A scaled duct then has the dimensions

a = \tilde{ξ} \tilde{a}, b = ξ \tilde{b}, a' = ξ \tilde{a}', b' = ξ \tilde{b}', and L_{D} = ξ {\tilde{L}}_{D},

with ξ being the scaling factor. As the spatial extension of the laser diodes is also upscaled one obtains

a_{d} = ξ {\tilde{a}}_{d} and b_{d} = ξ {\tilde{b}}_{d} .

Thus, all angles stay unchanged, especially

θ = \tilde{θ} and ϕ = \tilde{ϕ},

describing the equivalent surface. Therefore, the radii of the equivalent surface scale according to

r_{i} = ξ {\tilde{r}}_{i} and r_{0} = ξ {\tilde{r}}_{0}

and the emission characteristics of the diodes D and the solid angle d ²Ω_t scale by

D = \frac{1}{ξ^{2}} \tilde{D} and d^{2} Ω_{t} = d^{2} {\tilde{Ω}}_{t} .

Thus, the infinitesimal transmission d ² T_R scales like

d^{2} T_{R} = \frac{1}{ξ^{2}} d^{2} {\tilde{T}}_{R}

and the total transmisson efficiency changes to

\begin{array}{l} η_{T} & = & \int_{- \frac{b_{d}}{2}}^{\frac{b_{d}}{2}} d x_{d} \int_{- \frac{a_{d}}{2}}^{\frac{a_{d}}{2}} d z_{d} \int_{θ = - \frac{π}{2}}^{θ = \frac{π}{2}} \int_{ϕ = - π}^{ϕ = 0} d^{2} T_{R} \\ = & \int_{- \frac{b_{d}}{2}}^{\frac{b_{d}}{2}} d x_{d} \int_{- \frac{a_{d}}{2}}^{\frac{a_{d}}{2}} d z_{d} \int_{θ = - \frac{π}{2}}^{θ = \frac{π}{2}} \int_{ϕ = - π}^{ϕ = 0} \frac{1}{ξ^{2}} d^{2} {\tilde{T}}_{R} \\ = & \int_{- \frac{b_{d}}{2}}^{\frac{b_{d}}{2}} \frac{1}{ξ} d x_{d} \int_{- \frac{a_{d}}{2}}^{\frac{a_{d}}{2}} \frac{1}{ξ} d z_{d} \int_{\tilde{θ} = - \frac{π}{2}}^{\tilde{θ} = \frac{π}{2}} \int_{\tilde{ϕ} = - π}^{\tilde{ϕ} = 0} d^{2} {\tilde{T}}_{R} . \end{array}

By substitution of the variables using dx_d = ξdx̃_d and dz_d = ξdz̃_d we obtain

\begin{matrix} η_{T} & = & \int_{- \frac{{\tilde{b}}_{d}}{2}}^{\frac{{\tilde{b}}_{d}}{2}} d {\tilde{x}}_{d} \int_{- \frac{{\tilde{a}}_{d}}{2}}^{\frac{{\tilde{a}}_{d}}{2}} d {\tilde{z}}_{d} \int_{\tilde{θ} = - \frac{π}{2}}^{\tilde{θ} = \frac{π}{2}} \int_{\tilde{ϕ} = - π}^{\tilde{ϕ} = 0} d^{2} {\tilde{T}}_{R} \\ = & {\tilde{η}}_{T} . \end{matrix}

The transmission efficiency of the duct therefore stays unchanged under upscaling if the same scaling factor ξ is applied also to the diode array. This result is independent of reflection losses inside the duct. Therefore, under constant intensity the laser including pump delivery and diodes is completely up-scaleable at constant heat rate, with an interesting scaling law predicting that any geometrical dimension (despite the legnth of the laser medium) scales in proportionality to only the square-root of the laser power!

2.3. TIR-induced fluorescence effects

In order to realize an efficient quasi-three-level SSHCL a high pump and thus laser intensity is necessary inside the active medium [1]. High-power diode pumping can, however, not provide the high spatial brightness needed to pump an elongated medium. This arises from the fact that in order to achieve a homogenization and to obtain the pump intensities needed, the output of the pump diodes has to be compressed in cross section. Thus, due to etendue invariance, the angular distribution of the pump light is unavoidably increased.

To circumvent this problem, total-internal reflection inside the laser medium is used by polishing the lateral surfaces of the rod, keeping the high pump intensity along the laser rod. As shown in [15], this barrel polishing has an additional beneficial effect resulting in partial trapping of the medium’s own fluorescence. The reabsorbtion of this fluorescence by the laser medium itself then acts as additional pump light, thus increasing its efficiency. Another point of view to the same effect is that this reabsorbed fluorescence increases effectively the upper-state lifetime and therefore results in a lower effective laser threshold. Both descriptions result in the same effect: the upper-state lifetime can be increased from 7.6 ms to over 10 ms for Er³⁺:YAG. Therefore, instead of the spectrocopically expected 1.84 W/K increase in laser threshold a reduced 1.4 W/K increase is found as shown in Fig. 2.

Fig. 2 Evolution of laser threshold of a 5 mm diameter rod [15].

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3. Experimental results

The experimental setup of the kW-class laser is shown in Fig. 3. Electrically supplied by four batteries, four high power laser diode stacks emitting at 1532 nm each delivering between 2.5 kW to 3.2 kW of pump power are used to pump the Er³⁺:YAG rod. The hollow gold-coated ducts employed in this setup compress and homogenize the pump power symmetrically with a transmission efficiency of > 98%.

Fig. 3 Schematic of the experimental setup of the kW-class laser [16].

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An in-house developed high-current pulse generator is used and switches the current in the laser diodes on within 100 μs to a value up to 960 A of total current for any pulse duration between 200 μs and 10 s. A special current control circuit assures a constant pump current and thus constant pump power during the pulse. In a first experiment three different Er³⁺:YAG rod lengths from 120 mm to 160 mm are tested to determine the optimum absorption length, which itself depends on the laser operation conditions. These rods have a diameter of 10 mm and are doped with 0.25% Er³⁺. Flanged ends on the rods are used in order to capture all pump light exiting the 11.2 × 11.2 mm² exit window of the ducts. In the setup a laser rod is placed symmetrically inside a linear cavity consisting of a 5 m radius-of-curvature highly reflecting dielectric mirror (HR) and an 80% flat output coupler (OC) at a cavity length of approximately 126 cm. For true heat-capacity operation, the laser crystals were mounted in a way to minimize any heat transfer to the mount.

3.1. Short-pulse operation

To characterize the general laser efficiency free of any thermo-optical influence a series of experiments is performed at short pump pulse widths of 8 ms. Therefore, thermal inhomogeneities that can arise, e.g., from a non-homogeneous pump distribution inside the crystal are reduced to a great extent. In this pump time, comparable to the effective upper-state lifetime including fluorescence reabsorption expected for this type of laser geometry, up to 4.65 kW of output power are obtained with 11.3 kW of pump power, as can be seen in Fig. 4. The slope efficiency with respect to the incident pump power is 51.4% (58.4% when taken with respect to the absorbed pump power). Among the different crystals tested a length of 160 mm performes the best. Shorter crystals than 160 mm perform less, even though having lower reabsorption and thus lower thresholds, due to their decrease in pump absorption. A maximum in optical-to-optical efficiency of 43% is reached at around 10 kW of pump power. Also depicted in Fig. 4 are the output powers reached with different amounts of outcoupling for a 160 mm long crystal, showing that an outcoupling aroung 20% is a good value for this type of laser.

Fig. 4 Left: Output power and energy performance for 8 ms pump pulses at different crystal lengths. Right: Effect of the variation of the output coupler reflectivity on the laser performance for a L = 160 mm crystal.

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In order to investigate the scaling properties of the laser system, the performance of this kW-class laser is compared to previously obtained data on a smaller prototype using 5 mm-diameter rods [10]. By using the scaling law described in section 2.2 the output power data of a 5 mm-diameter rod with 100 mm in length and doped with 0.25% Er³⁺ are multiplied by a factor of 4 and shown together with the data from the kW-class laser in Fig. 5. These are compared to the results obtained with the 10 mm-diameter rod with 120 mm taking into account the slightly higher saturation of this rod. As can be seen, the two datasets precisely form one continuous curve, verifying the scalability of the laser in output power. Therefore, is can be stated that no physical principles would limit this laser in output power and thus in pulse energy. Only diode costs are an important point to take into account in future upscaling.

Fig. 5 Scaling performance verified by comparison with 5 mm-diameter rods.

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Concerning general scaling laws, however, the curve obtained with the 160 mm crystal shows that an even higher output power and efficiency could be obtained, corresponding to a dataset (or point of operation) which would not have been achieveable with the smaller system. Therefore, the scaling possibility can serve as source of confidence of what can be obtained by geometrical upscaling. However, in order to find the real optimum, a scaled system always needs to be experimentally realized and optimized in itself.

3.2. Long-pulse operation

Fig. 6 Pulse energy and temperature rise per pulse at various pulse durations for 11.3 kW of pump power.

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For a long pump pulse duration a build-up of thermal inhomogeneities occurs across the crystal. These inhomogeneities arise from the cylindrical shape of the crystal that focuses the pump light trapped by total-internal reflection tendentially onto the axis as shown in Fig. 7, thus creating a laterally non-uniform pump distribution causing a non-uniform heat deposition. This results in a transient phase distortion affecting the cavity stability and thus its mode distribution. Future research therefore will focus on pump homogeneity using non-cylindrical laser media to obtain the necessary homogeneous heat load for long operation times at high output power. First simulations, depicted in Fig. 7 show encouraging results of pump irradiation flatness achieved by simply passing to a square cross-section geometry.

Fig. 7 Simulated pump irradiation (in 10 kW/cm²) in the current cylindrical setup (10 mm diameter, left) and for the future setup using square cross-section crystals (here for 8 × 8 mm² for identical geometrical cross section, right). The plots from top to bottom show equidistant irradiance profiles from the entrance surface of the crystal towards the center. The concentration effect in the cylindrical geometry is clearly visible.

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In the current test cavity, existing of a 5 m radius-of-curvature mirror (HR) and a flat mirror (OC) at a separation of approximately 126 cm, a multimode operation results that optimizes the energy extraction from the active medium. However, this multimode pattern is very sensitive to phase distortions and therefore the extracted output power quickly decreases with time, as can be seen in Fig. 8. This measurement was performed by a large-scale integration sphere decoupling the output power measurement from eventual background signal arising from fluorescence or stray pump light; an effect that has to be taken into account at the high pump powers and corresponding fluorescence intensities.

Fig. 8 Measured laser output power for a fixed pulse duration of 0.8 s at 11.3 kW of pump power. The inset shows a measurement of 8 ms probe pulses fired during the cool-down phase of the crystal.

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Despite this transient thermal lens caused energy saturation effect a total pulse energy of up to 440 J is obtained by increasing the pump pulse width to 800 ms for the 160 mm rod. However, a linear increase in crystal temperature of 56.7 K/s is measured independently of the laser output power; a direct consequence of the moderate heat deposition in this laser architecture.

This transient thermal lensing effect in multimode operation is also a possible reason why a sequence of successive pulses, shown in Fig. 9, fired shortly enough after each other that the crystal temperature is effectively increased with each pulse, shows a less-than-expected thermal energy drop of 1.56 J/K. The spectroscopically expected value is around 3.1 J/K and may decrease to 2.33 J/K taking into accound the fluorescence reabsorption effect [17].

Fig. 9 Measured pulse energy of a series of pulses for a fixed pulse duration of 0.8 s at 11.3 kW of pump power. The inset shows the corresponding temperature evolution during this measurement.

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In order to correctly measure the influence of the true crystal temperature onto the laser performance without an influence from thermal inhomogeneities and thus to refute the common ideas that quasi-three-level lasers are unsuited for high-power heat-capacity operation due to thermal reabsorption, an 8 ms probe pulse was used during the cool-down phase of the laser crystal to determine the actual output power at a given crystal temperature. Therefore, the crystal was heated by several successive pulses of 800 ms to over 470 K. As the thermal time constant of the rod [18] is given by

τ_{th} = \frac{C ρ}{λ_{th}} R^{2} = 4.8 s,

wherein C is the specific heat, ρ the density and λ_th the thermal conductivity of YAG and R the rod radius, the probe pulses are separated by 30 – 40 s in order to allow the medium to equilibrate thermally between the pulses. As the pulse width itself is short enough to avoid the build-up of thermal inhomogeneities, the measured output power of each probe pulse can be seen as the true response of laser performance on temperature. This measurement, shown in the inset of Fig. 8, reveals a thermal power drop of 8.8 W/K, which is a very moderate value. Keeping in mind the temperature increase of 56.7 K/s at an output power at room temperature of 4.65 kW and assuming a solution is found that minimizes greatly the thermal inhomogeneities, e.g. by a proper pumping scheme and possibly using additional adaptive optics, an output energy of the current system of 14.6 kJ should be achievable in a 4 s pulse.

4. Conclusion

In conclusion, the spectroscopic fundametals, the scaling properties and experimental results of the quasi-three-level resonantly diode-pumped Er³⁺:YAG solid-state heat-capacity laser was described. Output powers of up to 4.65 kW are reached at very high optical-to-optical efficiencies of 43%. There results clearly refute the common ideas that quasi-three-level lasers are unsuited for high-power heat-capacity operation due to thermal reabsorption. Future research is dedicated to minimize thermal residual inhomogeneities and to increase the output pulse length to a timescale of several seconds.

Acknowledgments

The author thanks the WTD 91, Meppen, Germany for providing the d.c. energy source, J. Schöner and C. Maurer for mechanical drawings and design and T. Ibach, G. Stöppler and S. Bigotta (all of ISL) for support in the lab. Special thanks go to T. Ibach and S. Bigotta for the simulation of the pump irradiation for cylindrical and square geometries.

References and links

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4. M. Eichhorn, “Thermal lens effects in an Er³⁺:YAG laser with crystalline fiber geometry,” Appl. Phys. B 94, 451–457 (2009). [CrossRef]

5. G. Albrecht, S. Sutton, V. George, W. Sooy, and W. Krupke, “Solid state heat capacity disk laser,” Laser Part. Beams 16, 605–625 (1998). [CrossRef]

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8. M. Eichhorn, S. T. Fredrich-Thornton, E. Heumann, and G. Huber, “Spectroscopic properties of Er³⁺:YAG at 300 K–550 K and their effects on the 1.6 μm laser transitions,” Appl. Phys. B 91, 249–256 (2008). [CrossRef]

9. M. Eichhorn, “Numerical modeling of diode-end-pumped high-power Er³⁺:YAG lasers,” IEEE J. Quantum. Electron. 44(9), 803–810 (2008). [CrossRef]

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12. R. L. Boulanger, J.-L. Doualan, S. Girard, J. Margerie, and R. Moncorgé, “Excited-state absorption spectroscopy of Er³⁺-doped Y₃Al₅O₁₂, YVO₄ and phosphate glass,” Phys. Rev. B 60, 11380–11380 (1999). [CrossRef]

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16. M. Eichhorn, “High-efficiency multi-kilowatt Er³⁺:YAG solid-state heat-capacity laser,” Opt. Lett. 36, 1245–1247 (2011). [CrossRef] [PubMed]

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Multi-kW-class 1.64 μm Er³⁺:YAG lasers based on heat-capacity operation [Invited]

Abstract

1. Introduction

2. Theoretical background

2.1. Laser medium and spectroscopy

2.2. Pump delivery

2.3. TIR-induced fluorescence effects

3. Experimental results

3.1. Short-pulse operation

3.2. Long-pulse operation

4. Conclusion

Acknowledgments

References and links

Cited By

Figures (9)

Equations (9)

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